Many pharmaceutically active organic compounds can crystallize in different crystalline forms. That is, they can crystallize more than one type of molecular packing with more than one type of internal crystal lattice. The respective resulting crystal structures can have, for example, different unit cells. This phenomenon—identical chemical structure but different crystalline form—is referred to as polymorphism and the species having different molecular structures are referred to as polymorphs.
Many pharmacologically active organic compounds can also crystallize such that second, foreign molecules, especially solvent molecules, are regularly incorporated into the crystal structure of the principal pharmacologically active compound. This phenomenon is referred to as pseudopolymorphism and the resulting structures as pseudopolymorphs. When the second molecule is a solvent molecule, the pseudopolymorphs can be referred to as solvates.
However, it is generally not possible to predict whether a particular organic compound will form polymorphs or pseudopolymorphs, let alone predict the structure and properties of the polymorphs or pseudopolymorphs.
The discovery of a new crystalline form (polymorph or pseudopolymorph) of a pharmaceutically useful compound provides an opportunity to improve the performance characteristics of a pharmaceutical product. It enlarges the repertoire of materials that a formulation scientist has available for designing, for example, a pharmaceutical dosage form of a drug with a targeted release profile or other desired characteristic. It is clearly advantageous when this repertoire is enlarged by the discovery of new polymorphs or pseudopolymorphs of a useful compound. For a general review of polymorphs and the pharmaceutical applications of polymorphs see G. M. Wall, Pharm Manuf. 3, 33 (1986); J. K. Haleblian and W. McCrone, J. Pharm. Sci., 58, 911 (1969); and J. K. Haleblian, J. Pharm. Sci., 64, 1269 (1975), all of which are incorporated herein by reference.
Crystalline forms can be influenced by controlling the conditions under which the compound is obtained in solid form. Solid state physical properties that can differ from one crystalline form to the next include, for example, the flowability of the milled solid. Various crystalline forms can be more or less hygroscopic. Absorption of atmospheric moisture by a compound in powder form can impede its ability to flow. Flowability affects the ease with which the material is handled during processing into a pharmaceutical product. When particles of the powdered compound do not flow past each other easily, a formulation specialist must take that fact into account in developing a tablet or capsule formulation, which may necessitate the use of glidants such as colloidal silicon dioxide, talc, starch or tribasic calcium phosphate.
Another important solid state property of a pharmaceutical compound that can vary from one crystalline form to the next is its rate of dissolution in aqueous media, e.g. gastric fluid. The rate of dissolution of an active ingredient in a patient's stomach fluid can have therapeutic consequences since it imposes an upper limit on the rate at which an orally-administered active ingredient can reach the patient's bloodstream. The rate of dissolution is also a consideration in formulating syrups, elixirs and other liquid medicaments. The solid state form of a compound may also affect its behavior on compaction and its stability during storage.
These practical physical characteristics are influenced by the conformation, orientation, and packing of molecules in the unit cell, which characterize a particular polymorphic or pseudopolymorphic form of a substance. A polymorphic form may have thermodynamic properties different from those of the amorphous material or another polymorphic form. Thermodynamic properties can be used to distinguish between various polymorphs or pseudopolymorphs. Thermodynamic properties that can be used to distinguish between polymorphs and pseudopolymorphs can be measured in the laboratory by such techniques as capillary melting point, thermogravimetric analysis (TGA), differential scanning calorimetry (DSC), and differential thermal analysis (DTA).
A particular crystalline form can also possess distinct spectroscopic properties that may be detectable by, for example, solid state 13C NMR spectroscopy and infrared (IR) spectroscopy. This is particularly so in the case of solvates because of the presence of absorptions or resonances due to the second, foreign molecule.
(±)-1-Cyclopropyl-6-fluoro-1,4-dihydro-8-methoxy-7-(3-methyl-1-piperazinyl)-4-oxo-3-quinolenecarboxylic acid, commonly known as gatifloxacin, is a synthetic broad-spectrum antibacterial agent for oral or intravenous administration.
U.S. Pat. No. 5,880,283 discloses that gatifloxacin forms a hygroscopic hemihydrate. The hemihydrate (a pseudopolymorph) is reported to be easily formed upon crystallization of gatifloxacin from water-containing organic solvents. The hemihydrate reportedly has disadvantages for manufacturing of solid oral dosage forms, e.g. tablets. The patent further discloses a novel pseudopolymorph of gatifloxacin, the sesquihydrate, and presents thermal analysis and x-ray diffraction data for this crystalline form. The sesquihydrate is reported to be less hygroscopic and more stable in manufacturing.
U.S. Pat. No. 6,413,969 discloses at least 12 different polymorphs or pseudopolymorphs of gatifloxacin and discloses the x-ray powder diffraction diagrams of at least 10 of these. The hexahydrate, pentahydrate and sesquihydrate are crystallized directly from aqueous solvents. Other crystalline forms are crystallized from a molten phase or by solid-solid phase transformations. The pentahydrate form is, according to the disclosure of WO 02/22126 A1, the most thermodynamically stable form and has the lowest aqueous solubility at room temperature. The interrelationships between the twelve identified crystalline forms are given in the application.